The structure of a flat type vibration motor in the prior art is shown in FIGS. 1a and 1b, wherein, an eccentric vibrator 1 and winding coils a, b are bonded to a circuit board 2 with glue, and then are injection-molded with a center oil-retaining bearing 4 together to form a rotor 3; the center shaft 5 is fixed on a lower case 9, the rotor is mounted on the shaft through the bearing hole, and a magnet 8 is bonded to the lower case 9 to provide magnetic flux. A brush 10 is welded to a flexible circuit board 11, an upper case 7 covers the rotor and the lower case, and gaskets 6a, 6b are mounted on top side and bottom side of the rotor bearing respectively.
FIGS. 2a and 2b show the technical solution of a flat type vibration motor disclosed in China Patent Publication No. CN1206240A. In order to increase vibration amount, winding coils a, b, c are eccentrically mounted at one side, a piece of high-density composite engineering plastic 1 is mounted on the outer side of the eccentric rotor and protrudes from the bottom of the rotor so as to increase eccentric weight, wherein the protruding part is located in the circular space between the magnet 8 and the upper case 7. The middle part is made of a resin in density smaller than 1.8 Kg/m3.
FIGS. 3a and 3b show a technical solution for increasing vibration amount that is disclosed in Korea Patent Application No. 20-0333424, wherein the eccentric vibrator 1 is moved outwards to the edge of the rotor, and is bonded together with the winding coils to a hard circuit board 2, and then the rotor and the winding coils are injection molded together with the center bearing 4.
In the first technical solution described above, in order to prevent the vibrator from releasing, the vibrator is injection molded in plastic material, which limits the increase of the size of the vibrator and the eccentric distance.
In the second technical solution described above, since the eccentric weight is provided by the composite engineering plastic, the increase of eccentric weight is limited, and the size of the magnet has to be reduced greatly due to the protruding part of the rotor; as a result, the magnetic flux provided to the motor is reduced significantly, and the motor performance will be degraded severely.
Though the third technical solution can increase the eccentric amount to a certain degree, the increase in the size of the vibrator is very limited.
All the technical solutions described above have the drawbacks in that: the vibration amount will be reduced severely as the motor size is reduced; before the motor rotor is injection molded, the winding coils and the eccentric vibrator must be fixed by bonding, thus they may displace when the rotor is injection molded if they are not fixed well.